The principles of photothermal spectroscopy are generally described in a publication by Stephen E. Bialkowski entitled “Photothermal Spectroscopy Methods for Chemical Analysis”, John Wiley & Sons, Inc., 1996, the entire content of which is incorporated by reference herein. Photothermal spectroscopy method allows carrying out extremely sensitive measurements of optical absorption in homogeneous media. It is possible, using, a laser's coherent and powerful output, to obtain extremely sensitive measurements of optical absorption that exceed those of mass spectroscopy by two or three times, and produce accurate results from only a few molecules.
Co-pending U.S. patent application Pub. No. 20050105099 by the same inventor as the present application discloses implementation of coherent receiving technique for photothermal interferometric sensing. Integrated 90-degrees optical hybrid is a key component of the coherent receiver, This method provides improved sensitivity of the detection.
Laser free-space propagation is effected by atmospheric conditions such as turbulence and the like which works to cause aberrations in the spatial phase of the wavefront of the laser beams. The beam spreading and scintillation induced by the atmospheric turbulence cannot be compensated by increasing of optical power because of eye safety and power consumption. A considerable improvement is achieved by implementing adaptive optics systems such as described, for example, in U.S. Patent applications Nos. 20040086282 filed Oct. 16, 2003 by Graves (FIG. 4) and 20060024061 filed Feb. 2, 2006 by Wirth (See FIG. 3) and a number of publications, see for example, “Fiber coupling with adaptive optics for free space optical communications” by Weyrauch et al., Proceedings SPIE. 2002, v. 4489, p. 177-183, all of which incorporated herein by references.
In “Fiber coupling with adaptive optics for free space optical communications” by Weyrauch et al., Proceedings SPIE, 2002, v. 4489, p. 177-183 the laser beam coupling in multimode and single mode fiber is demonstrated. Obviously implementation of multimode fiber provides a number of advantages. First of all, larger sensitivity of the light beam detection is achieved because the diameter of multimode fiber is about 10 times larger than the diameter of single mode fiber. Secondly, the tolerance to the optical beam misalignment is attained since the diameter of multimode fiber is much larger than the focal spot size of a beam with uniform intensity distribution, while the focal spot size is compatible with a mode-field diameter of single-mode fiber.
There is a need for remote methods and systems for detecting for the presence of chemicals in the field which provide improved sensitivity due to elimination of atmospheric turbulence effects and improved coupling efficiency at the receiver.